https://tanguy.ortolo.eu/blog/article113/test-symlink en a blog about Debian and self-hosting Thu, 07 Nov 2013 19:28:00 +0000 PluXml https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383852486-1 https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383852486-1 If I may add, I guess any other option than `ln -sf` would end up being more expensive than simply forcing the symlinking. I wonder now if there are any reasons for *not* doing the '-sf' move, other than not looking like a hackish script. Thu, 07 Nov 2013 19:28:00 +0000 Victor Nițu https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383209230-1 https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383209230-1 @russ : Right, thanks for that information. I do not like it, since it may require to spawn additional processes, but most shells have test built-in, fortunately. Thu, 31 Oct 2013 08:47:00 +0000 Tanguy https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383176840-1 https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383176840-1 POSIX marks the "-a" option of test obsolescent; it would be better written as "] && [". Wed, 30 Oct 2013 23:47:00 +0000 russ https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383165251-1 https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383165251-1 I use readlink to do this kind of verfication: if ! readlink -e /target/file >/dev/null 2>&1 then ln -s /orig/file /target/file fi Wed, 30 Oct 2013 20:34:00 +0000 Guto https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383149075-1 https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383149075-1 @Patrick, @Zack Weinberg : Yes, in fact I forgot to mention the context. It is not in writing a generic piece of software, but a Debian package maintainer script, the symlink being to configure a web server for a web-based software. Here, it is not a matter of security, but only of avoiding a failure case if the user already put that link himself with a previous release. The user may indeed very well find a hard and tricky way to make the installation script fail, but that possibility is not relevant here, because there are already plenty of ways of doing that, the first one being to interrupt the installation with a ^C, a SIGTERM or a SIGKILL. Wed, 30 Oct 2013 16:04:00 +0000 Tanguy https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383148424-1 https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383148424-1 ... that should have been <code>perl -e 'symlink($ARGV[0], $ARGV[1]) or $!{EEXIST} or die "$ARGV[1]: $!\n"' /some/file /the/destination</code> Wed, 30 Oct 2013 15:53:00 +0000 Zack Weinberg https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383148370-1 https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383148370-1 Your "improved" code still has a bug: there's a gap between the check and the link creation, where another process could come in and create the file. Better to just create the link but allow it to fail: ln -s /some/file /the/destination 2> /dev/null || : or if you want the script to stop when some *other* kind of error pops up perl -e 'symlink($ARGV[0], $ARGV[1]) or $!{EEXIST} or die "$ARGV[1]: $!\n"' Wed, 30 Oct 2013 15:52:00 +0000 Zack Weinberg https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383148250-1 https://tanguy.ortolo.eu/blog/article113/test-symlink/#c1383148250-1 Please note, that the latter code is still incorrect, as it contains a race condition. It is triggered when externally creating the link after the check failed and before the script creates the link. As so often, it's really hard to get this right with a shell. Depending on your use case, "ln -sf" might be an option. Wed, 30 Oct 2013 15:50:00 +0000 Patrick